It is known in the art to adapt the speed of a cooling fan in an internal combustion engine to the required cooling power by means of a controllable clutch. The clutches are preferably fluid-filled friction clutches in which the degree of fluid filling is variable and which have a housing that is rotatable with respect to the drive assembly. The housing has a separating wall which defines a drive chamber and a fluid storage chamber and the drive chamber houses the drive disc. The separating wall between the two chambers has an opening that can be variably opened and closed by a valve assembly for the purpose of controlling the degree of fluid filling of the clutch. Clutches commonly in use employ a bimetallic element or an expansion element for actuating the valve. The valve-actuating forces are transmitted thereto by a piston or other mechanical linkage.
The clutch which carries the fan blades is located in the air stream that has passed through the engine radiator. Accordingly, the temperature-sensitive element in the clutch controls the operation of the clutch on the basis of the cooling air temperature which represents the coolant temperature.
A fluid coupling of this type is described, for example, in the German Pat. No. 1 284 186.
In recent times, engine designs have included the disposition of a separate radiator for cooling the supercharged air and locating this radiator ahead of the main water radiator. The modern designs also include the use of annular radiators having radial blowers, fans located in front of the radiators and blowers for air-cooled engines. In all of these special designs, the simple fluid friction coupling described is not applicable.
This is due to the fact that, in the first instance, the cooling air is additionally heated by passage through the super-charged air radiator and, in the other cases, the air is heated up only after passing through the fan. Accordingly, the variable clutch must be so constructed as to be controllable by the temperature of the engine coolant or some engine component and a control signal must be generated by a suitable thermal switch and transmitted to the clutch.
The medium for transmitting the signal may be compressed air, or electrical current. For example, a pneumatic control is known from U.S. Pat. No. 3,880,265. However, pneumatic and hydraulic control systems are expensive and require heavy and complicated conduits. The electrical signal transmission is substantially less expensive.
The requirement for rapid response of a fan clutch to temperature changes is admirably met by an electromagnetic control which receives its actuating signal from a thermal switch disposed at some critical location of the engine. Solenoid controls for fluid friction clutches are known for example from the German Auslegeschrift No. 1 270 339.
This publication describes a fluid clutch in which an electromagnet displaces an element out of the drive chamber, permitting the latter to be filled with oil. The annular electromagnet is stationary and is located in front of the clutch. Accordingly, a relatively great distance has to be provided between the clutch and the magnet to account for construction differences and oscillations. A further disadvantage of the known apparatus is that the magnet must be quite powerful so as to be able to move the displacement element against the force of a required return spring. Thus, the magnet involves a substantial expense and large weight and requires heavy electrical current. This latter property is especially disadvantageous because the operational safety requires that the clutch should engage when the electrical current fails. It is known that a fan coupling for motor vehicles is needed only during approximately 5% of the entire operation of the vehicle whereas, during the remaining 95%, the fan is disengaged, requiring that the electromagnet be powered. Accordingly, the energy consumption is substantial during most of the vehicle operation.